1. Field of the Invention
The present invention relates generally to a process for producing semiconductor devices and also to semiconductor devices produced according to the process. In particular, the present invention relates to a process for producing a Spin-on-Glass (SOG) film and also to an interlayer insulation film employing the SOG film.
2. Description of the Related Art
In order to realize higher integration of semiconductor integrated circuits, wiring must be much finer and multilayered. Interlayer insulation films are interposed between wiring layers so as to obtain multilayered wiring or interconnections. If the interlayer insulation films do not have flat surfaces, wiring layers formed on the insulation films are stepped, which leads to problems such as disconnection of wiring. Accordingly, the surfaces of the interlayer insulation films (i.e. the surfaces of the devices) must be as flat as possible. The technique of flattening the surface of a device is called planarization, which becomes more important as the wiring becomes finer and the number of layers increases.
SOG film is the type of interlayer insulation film which is most frequently employed in planarization. The use of SOG film has been researched and developed particularly with respect to planarization techniques utilizing flow characteristics of material for interlayer insulation films. In general, SOG means solutions of silicon-containing compounds in organic solvents as well as films that are formed from such solutions and that contain silicon dioxide as a major component. In forming an SOG film, a solution of a silicon-containing compound in an organic solvent is first dropped onto a substrate, which is in turn rotated. Then, the surface of the substrate is covered by the solution with recesses on the substrate filled by the solution. A wet silicon-containing film is thus formed such that the steps inherently formed on the substrate by wiring or interconnections may be compensated for. Thus, the surface of the substrate is planarized. Subsequently, the thus treated substrate is subjected to heat treatment to completely evaporate the organic solvent and promote polymerization reaction resulting in an SOG film having a flat surface.
SOG films include inorganic SOG films, in which the silicon-containing compounds contain no organic component, as represented by the following chemical formula (1): EQU [SiO.sub.2 ].sub.n (1), and
organic SOG films, in which the silicon-containing compounds contain organic components, as represented by the following chemical formula (2): EQU [R.sub.x SiO.sub.y ].sub.n (2)
wherein n, X and Y are integers; and R represents an alkyl group or an aryl group.
In general, inorganic SOG films involve disadvantages in that they are likely to contain water and hydroxyl groups in large amounts and that they are brittle compared with silicon oxide films formed by a Chemical Vapor Deposition (CVD) method and readily crack during heat treatment when the films have a thickness of 0.5 .mu.m or more. In contrast, organic SOG films have a molecular structure in which the linkages are partly blocked by alkyl or aryl groups. For this reason, cracking, which is liable to occur during heat treatment, can be controlled so that an organic SOG film having a thickness of about 0.5 .mu.m to 1.0 .mu.m is allowed to form. Accordingly, the use of organic SOG film permits not only the formation of a thick interlayer insulation film but also permits the planarization of the surface of a substrate having steps present thereon.
However, since organic SOG film contains organic components (i.e., hydrocarbon components), the rate of etching in defining via-contact holes is low where a mixed gas of carbon tetrafluoride and hydrogen (CF.sub.4 +H.sub.2) is used. To increase the etching rate, therefore, it is preferable to use a mixed gaseous system of carbon tetrafluoride and oxygen (CF.sub.4 +O.sub.2) in the etching treatment for forming via-contact holes in an organic SOG film. When such mixed gas of carbon tetraf luoride and oxygen is used as etching gas, however, a photoresist as an etching mask is unfortunately etched by the gas. As a result, the organic SOG film masked with the photoresist is also etched leading to the failure of accurate fine via-contact hole formation.
Organic SOG films also contain some water and hydroxyl groups, although the amounts are small compared with the case of inorganic SOG films. In general, the insulating properties and the mechanical strength of the SOG films are lower than those of the silicon oxide film formed by CVD method. Accordingly, when an SOG film is employed as an interlayer insulation film, a sandwich structure is often used where insulating films having high insulating property and high mechanical strength, in addition to the property of blocking water, are formed on upper and lower surfaces of the SOG film. Silicon oxide films formed by CVD process are usually employed as such additional insulating films.
However, since organic SOG film contains organic components, the organic SOG film is etched more than the upper and lower silicon oxide films are in the etching treatment for defining via-contact holes due to the water contained in the organic SOG film and the oxygen fed from the lower silicon oxide film. Further, in an ashing process, where the photoresist used as an etching mask is removed, the organic components contained in the organic SOG film are decomposed so that the organic SOG film is likely to shrink. Consequently, the organic SOG film cracks or retracts to form recesses therein. This is referred to as retrogression. If such recesses are formed, the via-contact holes cannot be fully filled with a wiring material, when wiring is to be formed by means of sputtering, resulting in the failure in securing excellent electric contact between two interconnections. Furthermore, when the organic components contained in the organic SOG film decompose, hygroscopicity of the organic SOG film is increased. These issues or topics are discussed in detail by C. K. Wang, L. M. Liu, H. C. Cheng, H. C. Huang and M. S. Lin in Proc. of IEEE VMIC, p.101 (1994).
Japanese Unexamined Patent Publication No. 1-307247 discloses a method in which an organic SOG film is subjected to O.sub.2 plasma treatment to convert the C--Si bond in the film to Si--O--Si bond, decomposing organic components contained in the organic SOG film. FIG. 1 shows IR absorption spectra of the organic SOG film before and after O.sub.2 plasma treatment. Incidentally, the organic SOG film has a film thickness of 3000 .ANG.. Referring to the method of forming the organic SOG film, a solution of the silicon-containing compound (CH.sub.3 Si(OH).sub.3) in ethanol is dropped onto a substrate, and the substrate is then rotated at 4800 rpm for 20 seconds to form a wet film of the solution on the substrate. Subsequently, the thus treated substrate is heat-treated successively at 100.degree. C. for one minute, at 200.degree. C. for one minute, at 300.degree. C. for one minute, at 22.degree. C. for one minute and at 300.degree. C. for 30 minutes in a nitrogen atmosphere to form an organic SOG film on the substrate. Next, the organic SOG film is subjected to an O.sub.2 plasma treatment for 60 seconds under the following conditions: RF power=500 W; preset temperature=360.degree. C.; oxygen flow rate=600 sccm.
In the IR absorption spectra of the organic SOG film shown in FIG. 1, the graphs 44-1, 44-2, 44-3 and 44-4 are spectra at the times: immediately after the formation of the SOG film (before the O.sub.2 plasma treatment); immediately after the O.sub.2 plasma treatment; after 3-day exposure under atmospheric condition in a clean room from the O.sub.2 plasma treatment; and after 7-day exposure under atmospheric condition in a clean room after the O.sub.2 plasma treatment, respectively.
As the graph 44-1 shows, absorption peaks attributed to organic components are observed around the wave numbers of about 3000 cm.sup.-1 and 1250 cm.sup.-1 before the O.sub.2 plasma treatment. The absorption peak around 3000 cm.sup.-1 is caused by the C--H bond stretching; whereas the absorption peak around 1250 cm.sup.-1 is caused by the C--H bond deformation or bending vibration. However, as the graphs 44-2 to 44-4 show, no absorption peak is observed around 3000 cm.sup.-1 and 1250 cm.sup.-1 after the O.sub.2 plasma treatment. Accordingly, it can be understood that the organic components contained in the organic SOG film are decomposed by the O.sub.2 plasma treatment.
However, as the graph 44-2 shows, absorption peaks attributed to hydroxyl groups are observed around 3600 cm.sup.-1 and 930 cm.sup.-1 immediately after the O.sub.2 plasma treatment. Generally, the absorption peak around 3600 cm.sup.-1 is caused by the O--H bond stretching in H--OH and Si--OH; whereas the absorption peak around 930 cm.sup.-1 is caused by the Si--O bond stretching in Si--OH. As the graphs 44-2 to 44-4 show, the absorption peaks around 3600 cm.sup.-1 and 930 cm.sup.-1 increase with time after the O.sub.2 plasma treatment. This is because the organic SOG film subjected to the O.sub.2 plasma treatment absorbs water in the atmosphere. Even if the organic SOG film is not subjected to the O.sub.2 plasma treatment, the film also absorbs water in the atmosphere, and the absorption peaks around 3600 cm.sup.-1 and 930 cm.sup.-1 increase with time. However, in the case of the organic SOG film subjected to the O.sub.2 plasma treatment, the increase in the absorption peaks is much more notable. As described above, the technique of applying an O.sub.2 plasma treatment to the organic SOG film involves the disadvantage that water and hydroxyl groups in the film increase, although it enjoys the advantage that organic components are decomposed.
Increase of the water and hydroxyl groups contained in the SOG film brings about defects such as "poisoned via phenomenon". The "poisoned via phenomenon" is a phenomenon where, when a metal is used for wiring, the wiring present in the via-contact holes is corroded by the water contained in the SOG film exposed to the via-contact holes. Further, it also happens that the water contained in the SOG film exposed to the via-contact holes reacts with the wiring filled in the via-contact holes to disadvantageously increase contact resistance. These issues or topics are discussed in detail by C. Chiang, N. V. Lam, J. K. Chu, N. Cox, D. Fraser, J. Bozarth and B. Mumford in Proc. of IEEE VMIC, p.404 (1987).
In order to overcome these problems, the above-described sandwich structure (where an SOG film is sandwiched by two silicon oxide films formed by CVD process) may be employed, and the SOG film may be etched back before the formation of the upper silicon oxide film. Thus, an inner wall of each via-contact hole can be composed of the upper and lower silicon oxide films only, with no exposed SOG film. However, the necessity of the step of etching back the SOG film complicates the production process, and thus lowers throughput.
Under such circumstances, one method has been proposed in which an organic SOG film is doped with fluorine by means of ion implantation to decompose organic components and also to reduce the water and hydroxyl groups contained in the SOG film (see L. J. Chen, S. T. Hsia and J. L. Leu, Proc. of IEEE VMIC, p.81 (1994)). This article describes that when fluorine ions were implanted at a dose of 3.times.10.sup.15 cm.sup.-2 to an organic SOG film having a film thickness of 4000 .ANG. with an acceleration energy of 40 keV or 80 keV, and the resulting SOG film was subjected to heat treatment at 425.degree. C. for 30 minutes, the water contained in the organic SOG film was reduced.
It has also been proposed to dope an organic SOG film with silicon or phosphorus by means of ion implantation so as to decompose organic components (see N. Moriya, Y. Shacham-Diamond and R. Kalish, J. Electrochem. Soc., Vol. 140, No. 5, p.1442 (1993)). Furthermore, a method has also been proposed in which organic components in an organic SOG film are decomposed by applying, for example, argon (Ar), nitrogen (N.sub.2) or nitrogen oxide (N.sub.2 O) plasma treatment to the SOG film (see C. K. Wang, L. M. Liu, H. C. Cheng, H. C. Huang and M. S. Lin, Proc. of IEEE VMIC, p.101 (1994); M. Matsuura, Y. Ii, K. Shibata, Y. Hayashide and H. Kotani, Proc. of IEEE VMIC, p.113 (1993)).
However, the method of doping an organic SOG film with fluorine by means of ion implantation involves the following disadvantages.
(1) When aluminum is used for wiring, the aluminum wiring may be corroded by the fluorine contained in the SOG film (see Jpn. J. Appl. Phys. Vol. 31, pp.2045-2048 Part 1, No.6B, June 1992).
(2) When a fluorine-doped SOG film is formed on a MOS transistor, the dielectric constant of a gate insulation film is lowered by the fluorine contained in the SOG film so that the effective thickness of the gate insulation film should be increased. Consequently, the designed characteristic values of the MOS transistor, e.g., threshold voltage, are changed.
(3) If a fluorine-doped SOG film is formed on a MOS transistor and subsequently source-drain regions of the transistor should be formed by diffusion of an impurity such as phosphorus or boron, the diffusion of the impurity may be inhibited by the fluorine-doped SOG film. Consequently, the designed characteristic values of the MOS transistor are changed.
(4) The method of doping an organic SOG film with a phosphorus or a phosphorus-containing compound by means of ion implantation involves the disadvantage, provided that aluminum is used for wiring, that the phosphorus reacts with the water contained in the SOG film to produce phosphoric acid (H.sub.3 PO.sub.4) which corrodes the aluminum wiring. Further, the method of doping an organic SOG film with silicon or a silicon-containing compound by means of ion implantation involves the disadvantage that conductivity of the SOG film is increased to exhibit deteriorated performance as interlayer insulation film.